Process to prepare borozirconate solution and use as a cross-linker in hydraulic fracturing fluids

ABSTRACT

A process to prepare a stable solution of a borozirconate complex is disclosed and use of the solution in oil field applications such as hydraulic fracturing and plugging of permeable zones. The process comprises contacting zirconium complex with a first alkanolamine, then water and optionally hydroxyalkylene diamine, then with a solution of a boron compound and a second alkanolamine. The solution is particularly suitable for use in a cross-linking composition in hydraulic fracturing and plugging of permeable zones of subterranean formations at temperatures of 275° F. (135° C.) and higher in the formation.

FIELD OF THE INVENTION

The present invention relates to borozirconate compositions and theiruse in oil field applications such as hydraulic fracturing and pluggingof permeable zones.

BACKGROUND OF THE INVENTION

The production of oil and natural gas from an underground well(subterranean formation) can be stimulated by a technique calledhydraulic fracturing, in which a viscous fluid composition (fracturingfluid) containing a suspended proppant (e.g., sand, bauxite) isintroduced into an oil or gas well via a conduit, such as tubing orcasing, at a flow rate and a pressure which create, reopen and/or extenda fracture into the oil- or gas-containing formation. The proppant iscarried into the fracture by the fluid composition and prevents closureof the formation after pressure is released. Leak-off of the fluidcomposition into the formation is limited by the fluid viscosity of thecomposition. Fluid viscosity also permits suspension of the proppant inthe composition during the fracturing operation. Cross-linking agents,such as borates, titanates or zirconates, are usually incorporated intothe fluid composition to control viscosity.

Typically, less than one third of available oil is extracted from a wellafter it has been fractured before production rates decrease to a pointat which recovery becomes uneconomical. Enhanced recovery of oil fromsuch subterranean formations frequently involves attempting to displacethe remaining crude oil with a driving fluid, e.g., gas, water, brine,steam, polymer solution, foam, or micellar solution. Ideally, suchtechniques (commonly called flooding techniques) provide a bank of oilof substantial depth being driven into a producing well; however, inpractice this is frequently not the case. Oil-bearing strata are usuallyheterogeneous, some parts of them being more permeable than others. As aconsequence, channeling frequently occurs, so that the driving fluidflows preferentially through permeable zones depleted of oil (so-called“thief zones”) rather than through those parts of the strata whichcontain sufficient oil to make oil-recovery operations profitable.

Difficulties in oil recovery due to thief zones may be corrected byinjecting an aqueous solution of an organic polymer and a cross-linkingagent into a subterranean formation under conditions where the polymerwill be cross-linked to produce a gel, thus reducing permeability of thesubterranean formation to the driving fluid (gas, water, etc.).Polysaccharide- or partially hydrolyzed polyacrylamide-based fluidscross-linked with certain aluminum, titanium, zirconium, and boron basedcompounds are used in these enhanced oil recovery applications.Cross-linked fluids or gels, whether for fracturing a subterraneanformation or for reducing permeability of zones in subterraneanformation, are now being used in hotter and deeper wells under a varietyof temperature and pH conditions. In these operations the rate ofcross-linking is critical to the successful generation of viscosity.

Boron-based compounds are typically used as cross-linkers in fracturingfluids utilized in low to mid temperature wells (150-250° F., 66-121°C.). Cross-linking takes place immediately on mixing of the boroncompound with the polymer base-gel. A pH of 10 or greater is required toinitiate cross-linking with boron-based cross-linkers. Because boroncross-linked gels are not shear sensitive, they can be used, even thoughthey cross-link at or near the surface.

Existing delayed zirconium-based cross-linkers, based on triethanolamineor hydroxyalkylated ethylenediamine have been designed to initiatecross-linking in the wellbore. Therefore, they are ineffective atgenerating viscosity under mild surface temperature conditions. The gelsare also shear sensitive and require higher horsepower (energyconsumption) to pump.

The need exists in some fracturing fluid applications to generate aninitial viscosity at the surface, followed by a delayed viscositygeneration, once the fluid is subjected to higher down-holetemperatures. In the case of mid-high temperature wells (250-300° F.,121-149° C.), a 3-8 minute delay in cross-linking is preferred. Fordeeper, higher temperature wells (300-400 ° F., 149-204° C.), it may benecessary to have cross-link times from 5-10 minutes.

Current technology involves using a borate-ion-generating-material incombination with a delayed zirconate cross-linker to accomplish bothsurface and delayed viscosity development. However, borate/zirconatecross-linking compositions suffer from disadvantages, such as poor shelfstability, insufficient viscosity generation and undesirablecross-linking rates.

U.S. Pat. No. 4,686,052 discloses a cross-linker comprising an organiczirconate stabilized with triethanolamine, optionally to which borax maybe added. The cross-linker mixture with borax has extremely longcross-linking time and low viscosity development.

There is a need for a borozirconate cross-linker which is stable onextended storage, is capable of generating excellent viscosity in thedesired 5-10 minute range for use in higher temperature wells, and whichcan be used in place of existing delayed zirconate cross-linkers inareas where an initial surface viscosity development is required, or inplace of delayed borate cross-linkers, which generally have limitedtemperature application. The present invention meets these needs.

SUMMARY OF THE INVENTION

The present invention provides a process for preparing a solution of aborozirconate complex suitable for use in a cross-linking compositionfor use as a fracturing fluid. The process comprises: (a) contacting azirconium complex with a first alkanolamine at a ratio of 2 to 10 molesof the first alkanolamine per mole of zirconium in an alcohol to form afirst mixture; (b) contacting the first mixture with water at a ratio ofabout 2 to 10 moles of water per mole of zirconium and with 0 to 1 molesof a hydroxyalkylene diamine per mole of zirconium to form a secondmixture; (c) contacting the second mixture with a solution of a boroncompound and a second alkanolamine in an alcohol, at a ratio of 1 to 4moles of boron per mole of zirconium and 1 to 4 moles of the secondalkanolamine per mole of boron and at a temperature of 25° C. to 90° C.for a period of time sufficient to stabilize the resulting borozirconatesolution.

There is further provided a process for preparing a solution of aborozirconate complex suitable for cross-linking in a fracturing fluidwhich consists of: (a) contacting a zirconium complex with a firstalkanolamine at a ratio of 2 to 10 moles of the alkanolamine per mole ofzirconium to form a first mixture; (b) contacting the first mixture withwater at a ratio of about 2 to 10 moles of water per mole of zirconiumto form a second mixture; (c) contacting the second mixture with asolution of a boron compound and a second alkanolamine in an alcohol, ata ratio of 1 to 4 moles of boron per mole of zirconium and 1 to 4 molesof the second alkanolamine per mole of boron and at a temperature of 25°C. to 90° C. for a period of time sufficient to stabilize the resultingborozirconate solution.

The present invention further provides a cross-linking compositioncomprising a solution of a borozirconate complex prepared according tothe process of this invention and a method to use the cross-linkingcomposition as a fracturing fluid.

DETAILED DESCRIPTION OF THE INVENTION

Trademarks and tradenames are shown herein in upper case.

This invention provides a solution of borozirconate complex suitable forcross-linking a fracturing fluid which is stable on extended storage andis capable of generating excellent viscosity in the desired 5-10 minuterange for use in higher temperature wells. By “stable” it is meant asolution comprising borozirconate complex prepared according to theprocess of this invention can be stored at ambient temperature for atleast six months without precipitation. The borozirconate complex can beadvantageously used in place of existing delayed zirconate cross-linkersin areas where an initial surface viscosity development is required, orin place of delayed borate cross-linkers, which generally have limitedtemperature application.

The solution of borozirconate complex is provided by a processcomprising: (a) contacting a zirconium complex with a first alkanolamineat a ratio of 2 to 10 moles of the first alkanolamine per mole ofzirconium in an alcohol to form a first mixture; (b) contacting thefirst mixture with water at a ratio of about 2 to 10 moles of water permole of zirconium and with 0 to 1 moles of a hydroxyalkylene diamine permole of zirconium to form a second mixture; (c) contacting the secondmixture with a solution of a boron compound and a second alkanolamine inan alcohol, at a ratio of 1 to 4 moles of boron per mole of zirconiumand 1 to 4 moles of the second alkanolamine per mole of boron and at atemperature of 25° C. to 90° C. for a period of time sufficient tostabilize the resulting borozirconate solution. Preferably, the firstmixture is contacted with water and a hydroxyalkylene diaminesimultaneously in step (b). Alternatively the first mixture is contactedwith a hydroxyalkylene diamine prior to or after water addition.Preferably, the first mixture is contacted with water and ahydroxyalkylene diamine at a ratio of 0.1 to 1 mole of hydroxyalkylenediamine per mole of zirconium, more preferably at a ratio of 0.5 to 1mole of hydroxyalkylene diamine per mole of zirconium.

The first mixture in step (a), an alcoholic solution of a zirconiumcomplex with an alkanolamine, can be prepared by a process whichcomprises contacting a solution of a tetraalkyl zirconate in a C₁-C₆alcohol with from 2 to 10 moles molar equivalents of an alkanolamine permole of zirconium.

A number of tetraalkyl zirconates (also known as zirconiumtetraalkoxides) can be used to prepare the above zirconium complex,e.g., tetra-isopropyl zirconate, tetra-n-propyl zirconate, andtetra-n-butyl zirconate. The preferred tetraalkyl zirconate istetra-n-propyl zirconate, available as TYZOR NPZ organic zirconate, asolution in n-propanol, with a zirconium content as ZrO₂ of about 28% byweight, and available from E. I. du Pont de Nemours and Company,Wilmington, Del.

Examples of suitable alkanolamines include, but are not limited to,triethanolamine, tri-n-propanolamine, triisopropanolamine,diisopropanolamine, and their mixtures. Preferably the alkanolamine istriethanolamine.

Contacting the above tetraalkyl zirconates with the alkanolamine can becarried out at a variety of temperatures, e.g., between 25° C. and 90°C., preferably between 50° C. and 80° C., and in any order. The firstmixture is then held at this temperature for a sufficient period toreach equilibrium. We have found about 2 hours at 60° C. to be adequate,but other periods and temperatures may also be used.

In step (b), the first mixture is contacted with water at the rate ofabout 2 to 10 moles of water per mole of zirconium. Optionally, thefirst mixture is also contacted with a hydroxyalkylene diamine in thisstep (b) at a ratio of 0.1 to 1 moles of hydroxyalkylene diamine permole of zirconium. The hydroxyalkylene diamine acts as a chelatingagent, allowing one to adjust the rate of cross-linking by increasing ordecreasing the amount added. Preferably the hydroxyalkylene diamine istetra-hydroxyisopropylethylenediamine (QUADROL). The second mixture isthen held at a temperature between 25° C. and 90° C. for a period oftime sufficient to reach equilibrium. A contact time of about 2 hours at60° C. is adequate, but other periods and temperatures may also be used.

In step (c), the second mixture from step (b) is contacted with a boroncompound, a second alkanolamine and alcohol at a ratio of 1 to 4 molesof the boron compound per mole of zirconium and at a ratio of 1 to 4moles of second alkanolamine per mole of boron.

The contacting step is performed in an alcohol at a temperature of 25°C. to 90° C. for a period of time sufficient to stabilize the resultingsolution. A contact time of about 2 hours at 60° C. is adequate, butother periods and temperatures may also be used.

The boron compound, the second alkanolamine and alcohol may be contactedseparately and in any order, or as a solution of any of the twoingredients with the third added separately, or as a solution of allthree ingredients, with the second mixture. Preferably, the secondmixture is a solution of all three ingredients, i.e., the boroncompound, the second alkanolamine and alcohol, is added to the secondmixture. Advantageously combining the second alkanolamine with the boroncompound in an alcohol provides a solution, which is easier to handlethan a boron slurry (which is formed absent the alkanolamine). Theamount of alcohol may be any amount that results in a clear solution.

The boron compound may be selected from the group consisting of boricacid and trialkyl borates. Preferably the boron compound is boric acid.The alkanolamine solubilizes the boron compound, especially boric acid,allowing boron to be added as a solution, rather than a solid or as aslurry.

The second alkanolamine can be the same or different from the firstalkanolamine and is typically selected from the group consisting oftriethanolamine, tripropanolamine, triisopropanolamine, anddiisopropanolamine. Preferably the second alkanolamine istriethanolamine.

The alcohol solvent may be methanol, isopropanol, n-propanol or otheralcohol with up to six carbon atoms. Preferably the alcohol is methanol,isopropanol, or n-propanol.

The present invention also provides a cross-linking composition whichcomprises an aqueous liquid; a pH buffer; a cross-linkable organicpolymer; and a solution of a borozirconate made by a process comprising(a) contacting a zirconium complex with a first alkanolamine at a ratioof 2 to 10 moles of the first alkanolamine per mole of zirconium in analcohol to form a first mixture; (b) contacting the first mixture withwater at a ratio of about 2 to 10 moles of water per mole of zirconiumand with 0 to 1 moles of a hydroxyalkylene diamine per mole of zirconiumto form a second mixture; (c) contacting the second mixture with asolution of a boron compound and a second alkanolamine in an alcohol, ata ratio of 1 to 4 moles of boron per mole of zirconium and 1 to 4 molesof the second alkanolamine per mole of boron and at a temperature of 25°C. to 90° C. for a period of time sufficient to stabilize the resultingborozirconate solution. Preferably, the hydroxyalkylene diamine is addedwith the water in step (b) at a mole ratio of 0.1 to 1 mole ofhydroxyalkylene diamine per mole of zirconium.

The aqueous liquid is typically selected from the group consisting ofwater, aqueous alcohol, and aqueous solution of a clay stabilizer. Thealcohol can be the same or different alcohol as the reaction solvent,that is, an alcohol having 1 to 6 carbon atoms. Preferably, when theaqueous liquid is aqueous alcohol, the alcohol is methanol or ethanol.Clay stabilizers include, for example, hydrochloric acid and chloridesalts, such as, tetramethylammonium chloride (TMAC) or potassiumchloride. Aqueous solutions comprising clay stabilizers may comprise,for example, 0.05 to 0.5 weight % of the stabilizer, based on thecombined weight of the aqueous liquid and the organic polymer (i.e., thebase gel). Preferably, when the aqueous liquid is an aqueous solution ofa clay stabilizer, the clay stabilizer is tetramethylammonium chlorideor potassium chloride.

The aqueous liquid can also be a mixture of water and one or moreorganic solvents. Organic solvents that may be used include alcohols,glycols, polyols, and hydrocarbons such as diesel.

Preferably, the aqueous liquid is water, aqueous methanol, aqueousethanol, an aqueous solution of potassium chloride, an aqueous solutionof tetramethylammonium chloride, or a combination of two or morethereof.

The cross-linking composition comprises an effective amount of a pHbuffer to control pH. The pH buffer may be acidic, neutral or basic. ThepH buffer is generally capable of controlling the pH from about pH 5 toabout pH 12. For example in a composition for use at a pH of 5-7, afumaric acid-based buffer or a sodium diacetate-based buffer can beused. In a composition for use at a pH of 7-8.5, a sodiumbicarbonate-based buffer can be used. In a composition for use at a pHof 9-12, a sodium carbonate or sodium hydroxide-based buffer can beused. Other suitable pH buffers can be used, as are known to thoseskilled in the art.

The composition further comprises a cross-linkable organic polymer.Suitable cross-linkable organic polymers are selected from the groupconsisting of solvatable polysaccharides, polyacrylamides andpolymethacrylamides. Preferably the organic polymer is a solvatablepolysaccharide and is selected from the group consisting of gums, gumderivatives and cellulose derivatives. Gums include guar gum and locustbean gum, as well as other galactomannan and glucomannan gums, such asthose derived from sennas, Brazilwood, tera, honey locust, karaya gumand the like. Preferred gum derivatives include hydroxyethylguar (HEG),hydroxypropylguar (HPG), carboxyethylhydroxyethylguar (CEHEG),carboxymethylhydroxypropylguar (CMHPG), and carboxymethyl guar (CMG).Preferred cellulose derivatives include those containing carboxylgroups, such as carboxymethylcellulose (CMC) andcarboxymethylhydroxyethylcellulose (CMHEC). The solvatablepolysaccharides can be used individually or in combination; usually,however, a single material is used. Guar derivatives and cellulosederivatives are preferred, such as, HPG, CMC and CMHPG. HPG is generallymore preferred based upon its commercial availability and desirableproperties. However, CMC and CMHPG may be more preferred incross-linking compositions when the pH of the composition is less than6.0 or higher than 9.0, or when the permeability of the formation issuch that one wishes to keep the residual solids at a low level toprevent damage to the formation. The cross-linkable polymer is normallymixed with the aqueous liquid to form a base gel.

The solution of borozirconate complex is prepared as describedpreviously, and may contain an added solvent or solvents.

The cross-linking composition may comprise optional components,including those which are common additives for oil field applications.Thus, the composition may further comprise one or more of proppants,friction reducers, bactericides, hydrocarbons, chemical breakers,polymer stabilizers, surfactants, formation control agents, and thelike. Proppants include sand, bauxite, glass beads, nylon pellets,aluminum pellets and similar materials. Friction reducers includepolyacrylamides. Hydrocarbons include diesel oil. Chemical breakersbreak the cross-linked polymer (gel) in a controlled manner and includeenzymes, alkali metal persulfate, and ammonium persulfate. Polymerstabilizers include methanol, alkali metal thiosulfate, and ammoniumthiosulfate.

These optional components are added in an effective amount sufficient toachieve the desired cross-linking performance based on the individualcomponents, desired cross-linking time, temperature and other conditionspresent in the formation being fractured or permeable zone beingplugged.

The cross-linking composition is produced by mixing the solution of theborozirconate complex with the other components, in any order. Forexample, in one particular application in an oil field, the solution ofborozirconate complex and optional components are introduced into aformation, while the cross-linkable organic polymer and aqueous liquidare introduced into the formation as a separate stream. The pH buffer isindependently admixed with the zirconium solution, the organic polymerand/or the aqueous liquid. Alternatively, all components may be premixedand introduced into a subterranean formation as a single stream.Advantageously, the components may be mixed in different combinations,and more advantageously, the components may be mixed just prior to useto enable easy variation and adjustment of the cross-linking rate.

This invention provides a method for hydraulically fracturing asubterranean formation, which comprises introducing into the formationat a flow rate and pressure sufficient to create, reopen, and/or extendone or more fractures in the formation, a cross-linking compositioncomprising an aqueous liquid; a pH buffer; a cross-linkable organicpolymer, and a solution of a borozirconate complex described previously,and made by a process comprising (a) contacting a zirconium complex witha first alkanolamine at a ratio of 2 to 10 moles of the firstalkanolamine per mole of zirconium in an alcohol to form a firstmixture; (b) contacting the first mixture with water at a ratio of about2 to 10 moles of water per mole of zirconium and with 0 to 1 moles of ahydroxyalkylene diamine per mole of zirconium to form a second mixture;(c) contacting the second mixture with a solution of a boron compoundand a second alkanolamine in an alcohol, at a ratio of 1 to 4 moles ofboron per mole of zirconium and 1 to 4 moles of the second alkanolamineper mole of boron and at a temperature of 25° C. to 90° C. for a periodof time sufficient to stabilize the resulting borozirconate solution.Preferably, the hydroxyalkylene diamine is added with the water in step(b) of preparing the solution of borozirconate complex at a mole ratioof 0.1 to 1 mole of hydroxyalkylene diamine per mole of zirconium.

In one embodiment of the method for hydraulically fracturing asubterranean formation, the solution of borozirconate complex and thecross-linkable polymer are contacted prior to their introduction intothe formation, such that the cross-linking agent and polymer react toform a cross-linked gel. The gel is then introduced into the formationat a flow rate and pressure sufficient to create, reopen, and/or extenda fracture in the formation.

In this method, a base gel is prepared by mixing a cross-linkableorganic polymer with an aqueous liquid. Then the cross-linked gelcomposition is prepared by mixing the base gel with a solution of theborozirconate complex described previously, and made by a processcomprising contacting an alcoholic solution of a zirconium complex of analkanolamine with water and a hydroxyalkylene diamine, and then a boroncompound and an alkanolamine in alcohol at a temperature of 25° C. to90° C. for a period of time sufficient to stabilize the resultingsolution, wherein 2 to 10 moles of alkanolamine and 2 to 10 moles ofwater per mole of zirconium and 0 to 1 mole, preferably 0.1 to 1 mole ofhydroxyalkylene diamine are added with the zirconium complex, followedby 1 to 4 moles of the boron compound and 1 to 4 moles of analkanolamine per mole of zirconium. More preferably, the hydroxyalkylenediamine is added with water at the rate of 0.5 to 1 moles ofhydroxyalkylene diamine per mole of zirconium in the process ofpreparing the solution of borozirconate complex. The solution ofborozirconate complex, the base gel, or both further comprise a pHbuffer.

Alternatively, the subterranean formation may be penetrated by awellbore, such that contacting the solution of borozirconate complexwith the base gel occurs in the wellbore and the cross-linked gel isintroduced into the formation from the wellbore. This method ofhydraulically fracturing a subterranean formation penetrated by awellbore comprises (a) preparing a base gel by mixing a cross-linkableorganic polymer with an aqueous liquid; (b) introducing the base gelinto the wellbore; (c) simultaneously with or sequentially afterintroducing the base gel into the wellbore, introducing the solution ofborozirconate complex described previously, and made by a processcomprising (1) contacting a zirconium complex with a first alkanolamineat a ratio of 2 to 10 moles of the first alkanolamine per mole ofzirconium in an alcohol to form a first mixture; (2) contacting thefirst mixture with water at a ratio of about 2 to 10 moles of water permole of zirconium and with 0 to 1 moles of a hydroxyalkylene diamine permole of zirconium to form a second mixture; (3) contacting the secondmixture with a solution of a boron compound and a second alkanolamine inan alcohol, at a ratio of 1 to 4 moles of boron per mole of zirconiumand 1 to 4 moles of the second alkanolamine per mole of boron and at atemperature of 25° C. to 90° C. for a period of time sufficient tostabilize the resulting borozirconate solution; (d) permitting the basegel and the solution of borozirconate complex to react to form across-linked aqueous gel; and (e) introducing the cross-linked gel intothe formation from the wellbore at a flow rate and pressure sufficientto create, reopen, and/or extend a fracture in the formation.Preferably, the hydroxyalkylene diamine is added with the water at aratio of 0.1 to 1 mole of hydroxyalkylene diamine per mole of zirconium,more preferably at a ratio of 0.5 to 1 mole of hydroxyalkylene diamineper mole of zirconium. A pH buffer is independently admixed with thebase gel, the solution of borozirconate complex or both prior tointroducing the base gel and the borozirconate solution into thewellbore.

Upon creation of a fracture or fractures, the method may furthercomprise introducing a cross-linking composition comprising the solutionof borozirconate complex, a cross-linkable organic polymer and proppantinto the fracture or fractures. This second introduction of a solutionof borozirconate complex is preferably performed in the event thecross-linking composition used to create the fracture or fractures didnot comprise proppant.

Another use for the solution of borozirconate complex of the presentinvention relates to a method for selectively plugging permeable zonesand leaks in subterranean formations which comprises introducing intothe permeable zone or the site of the subterranean leak, a cross-linkingcomposition comprising (a) an aqueous liquid; (b) a pH buffer, (c) across-linkable organic polymer; and (d) an aqueous solution of theborozirconate complex described previously. The pH buffer may be admixedwith the solution of borozirconate complex prior to introducing thecross-linking composition into the permeable zone or site of the leak.

In a first embodiment of the method for plugging a permeable zone or aleak in a subterranean formation, the aqueous liquid, pH buffer,cross-linkable organic polymer and the solution of borozirconate complexare contacted prior to their introduction into the subterraneanformation, such that the polymer and borozirconate complex react to forma cross-linked aqueous gel, which gel is then introduced into theformation.

In an alternative embodiment of the method for plugging a permeable zoneor a leak in a subterranean formation, the solution of borozirconatecomplex and the cross-linkable organic polymer are introducedseparately, either simultaneously or sequentially, into the permeablezone or the site of the subterranean leak such that cross-linking occurswithin the subterranean formation. This method comprises (a) preparing abase gel by mixing a cross-linkable organic polymer with an aqueousliquid; (b) introducing the base gel into the into the permeable zone orthe site of the subterranean leak, (d) simultaneously with orsequentially after, introducing the base gel into the into the permeablezone or the site of the subterranean leak, introducing the solution ofborozirconate complex into the permeable zone or the site of thesubterranean leak; (e) permitting the base gel and the cross-linkingagent to react to form a cross-linked aqueous gel to plug the zoneand/or leak. The solution of borozirconate complex, the base gel, orboth further comprise a pH buffer.

The relative amounts of cross-linkable organic polymer and theborozirconate complex may vary. One uses small but effective amountswhich for both will vary with the conditions, e.g., the type ofsubterranean formation, the depth at which the method (e.g., fluidfracturing, permeable zone plugging or leak plugging) is to beperformed, temperature, pH, etc. Generally one uses as small an amountof each component as will provide the viscosity level necessary toeffect the desired result, i.e., fracturing of the subterraneanformation, or plugging permeable zones or leaks to the extent necessaryto promote adequate recovery of oil or gas from the formation.

For example, satisfactory gels can generally be made for fluidfracturing by using the cross-linkable organic polymer in amounts up toabout 1.2 weight % and the cross-linking composition in amounts up toabout 0.50 weight % of the borozirconate complex, with percentages beingbased on the total weight of the cross-linking composition. Preferably,from about 0.25 to about 0.75 weight % of the cross-linkable organicpolymer is used and from about 0.05 to about 0.25 weight % of theborozirconate complex is used.

In a method for plugging permeable zones or leaks, generally about 0.25to 1.2 weight % of a cross-linkable organic polymer is used, preferably0.40 to 0.75 weight %, based on the total weight of the cross-linkingcomposition. Generally about 0.01 to 0.50 weight % of the borozirconatecomplex is used, preferably 0.05 to 0.25 weight %, based on the totalweight of the cross-linking composition.

The amount of borozirconate complex used to cross-link the organicpolymer is that which provides a zirconium ion concentration in a rangefrom about 0.0005 weight % to about 0.1 weight %, based on the totalweight of the cross-linking composition. The preferred concentration ofzirconium ion is in the range of from about 0.001-0.05 weight %, basedon the total weight of the cross-linking composition.

Typically the solution of borozirconate complex of this invention can beused at a pH of from about 8 to 11. Advantageously, the solution ofborozirconate complex of this invention is used at a temperature of275-325° F. (135-163° C.). For successful completion of the fracturingoperation, whether hydraulic fracturing or plugging a permeable zone,the cross-linking composition should provide a viscosity of at least 200centipoise (Cp), preferably at least 300 Cp, 90 minutes afterintroducing the cross-linking composition into the subterraneanformation or permeable zone or site of a subterranean leak.

EXAMPLES

The preparation of the compositions in the Comparative Examples and inthe Examples were each carried out in closed vessels containing anagitator, thermometer, condenser, nitrogen inlet and dropping funnel.Unless specified otherwise, percentages are given by weight.Temperatures are given in degrees Celsius. The cross-linking propertiesof the Comparative Example and Example compositions are provided as afunction of the viscosity of carboxymethylhydroxypropylguar cross-linkedwith the compositions of the Comparative Example and Example.

Preparation of Base Gel

A Waring blender jar was filled with 1 liter of distilled water. To thiswas added 2 g of a 50% aqueous solution of tetramethylammonium chlorideclay stabilizer. Agitation was started and 3.6 g ofcarboxymethylhydroxypropylguar (CMHPG) was sprinkled into the vortex ofthe agitating solution. The pH of the resultant slurry was adjusted to 6with sodium diacetate and agitation continued for 30 minutes. The pH wasthen adjusted to 10.3 with 10% sodium hydroxide solution. Agitation wasstopped and the gel was allowed to stand for 30 minutes or more beforeuse.

Viscosity Measurement of Zirconate Cross-Linked Base Gel

To 250 ml of a vigorously agitated sample of base gel in a Waringblender jar, was added 0.00032 moles of zirconium (0.2-1.0 ml dependenton percent zirconium of cross-linker solution—hereinafter referred to asthe Standard Loading Density), for each Comparative Example A-C andExample 1-7. Agitation was continued for about 15-180 seconds. A 25 mlsample of the cross-linker containing gel was placed in the cup of theFANN 50 Viscometer with an R-1, B-3 configuration and viscosity wasmeasured at 275° F. (135° C.) and 122 rpm at 100 reciprocal seconds ofshear.

The following Comparative Examples are based on the range of componentmolar ratios disclosed in U.S. Pat. Nos. 4,686,052 and 4,514,309 andBritish Patent No. GB 2,108,122. For comparison purposes, testingconditions used to determine cross-linking efficiency were the same asused in the test conditions for the solutions borozirconate complexprepared according to the process of this invention. The test conditionsdiffer slightly from those used in the aforementioned U.S. Patents,particularly in that carboxymethylhydroxypropylguar (CMHPG) was used inthese tests rather than hydroxypropylguar as previously used. CMHPG isthe preferred polymer for use by service companies with zirconate-basedcross-linkers for high pH, high temperature applications. Results areprovided in Table 1.

Comparative Example A

A 500-ml flask was charged with 10.4 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate) and 32.2 g of n-propanol. Agitation was startedand 11.2 g of triethanolamine were added. The resulting mixture washeated to 60° C. and held at this temperature for 2 hours. Then, amixture of 21.4 g of water and 136.4 g of triethanolamine was added.When addition was complete, another 97.8 g of water were added followedby 3.5 g of sodium tetraborate. This mixture was heated for another hourat 60° C. and then cooled to room temperature to give 313 g of a paleyellow liquid containing 0.67% Zr and 0.6% B.

Comparative Example B

A 500-ml flask was charged with 10.4 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate) and 24.1 g of n-propanol. Agitation was startedand 22.4 g of triethanolamine were added. The resulting mixture washeated to 60° C. and held at this temperature for 2 hours. Then, amixture of 21.4 g of water and 136.4 g of triethanolamine was added.When addition was complete, another 97.8 g of water were added followedby 3.5 g of sodium tetraborate. This mixture was heated for another hourat 60° C. and then cooled to room temperature to give 316 g of a paleyellow liquid containing 0.67% Zr and 0.6% B.

Comparative Example C

A 500-ml flask was charged with 48.2 g of sodium zirconium lactate(TYZOR 217 organic zirconate) and 20 g of tetra-triethanolaminezirconate (TYZOR TEAZ organic zirconate). Agitation was started and 22.4g of triethanolamine were added. The resulting mixture was heated to 60°C. and held at this temperature for 2 hours. Then, a mixture of 5 g ofboric acid and 66.7 g of methanol was added. This mixture was heated foranother hour at 60° C. and then cooled to room temperature to give 140 gof a pale yellow liquid containing 1.9% Zr and 0.63% B.

Test results for Comparative Examples are provided in Table 1 below. Theabbreviations and headings used in Tables 1, 2 and 3 are as follows. The% Zr is the percent of zirconium in the solution prepared in thecorresponding Comparative Example or Example; ml refers to themilliliters of cross-linking solution injected in the test. NPZ is TYZORNPZ organic zirconate; 217 is TYZOR 217 organic zirconate; TEAZ is TYZORTEAZ organic zirconate; TEA is triethanolamine; Polyol ishydroxyisopropylethylenediamine, QUADROL polyol; B.A. is boric acid; napis n-propanol; MA is methanol. The amounts of each component added isgiven in grams, g. The values in parentheses, which follow the amounts,refer to molar ratio of the component compared to zirconium. Note thatthe mole ratio for zirconium is 1.

“Fann Time Max, min.” means the time, in minutes, it takes to reachmaximum viscosity. The viscosity at this maximum time is labeled “Cp @Max.”, to indicate viscosity in centipoise (Cp). The viscosity after 90minutes at the test temperature is labeled “Cp @90 min.”

TABLE 1 Compositions and Performance of Comparative Examples Fann Comp.NPZ, g 217, g TEAZ, g First TEA, g B.A., g Alcohol, g Second TEA, g TimeCp @ Cp @ Example % Zr (mole ratio) (mole ratio) (mole ratio) (moleratio) (mole ratio) (Alcohol) (mol ratio) Max, min. Max. 90 min. A 0.7010.4 (1) 11.2 (3) 3.5 (1.6) 32.2 136.4 (38) 24 314 228 (nPA) B 0.70 10.4(1) 22.4 (6) 3.5 (1.6) 24.1 136.4 (38) 14 356 190 (nPA) C 1.90 48.2(0.5) (0.5)   22.4 (5.2)   5 (2.8) 66.7 6 536 150 (MA)

The data in Table 1 show that under these testing conditions, theComparative Examples cross-linked at much too slow a rate (>8 minutes)and/or generated insufficient viscosity (<200 Cp) to allow successfulcompletion of the fracturing operation. Comparative Examples A and Bcross-linked much too slowly to be of practical use under fieldconditions typically encountered. Comparative Example C did cross-linkin the desired 3-10 minute range; however viscosity retention after 90minutes was less than 200 Cp.

The following Examples show the inventive process to prepareborozirconate solutions and results of use of the solutions ascross-linkers.

Example 1

A 500-ml flask was charged with 167 g of tetra-triethanolamine zirconate(TYZOR TEAZ organic zirconate). Agitation was started and the reactionwas heated to 60° C. 33 g of water were then added. The resultingmixture was held at 60° C. for 2 hours. Then, a mixture of 30 g of boricacid, 72 g of triethanolamine and 65.4 g of methanol was added. Thismixture was heated for another hour at 60° C. and then cooled to roomtemperature to give 367 g of a pale yellow liquid containing 6% Zr and1.45% B.

Example 2

A 500-ml flask was charged with 151 g of tetra-triethanolamine zirconate(TYZOR TEAZ organic zirconate). Agitation was started and the reactionwas heated to 60° C. 29.9 g of water were then added. The resultingmixture was held at 60° C. for 2 hours. Then, a mixture of 40.7 g ofboric acid, 97.7 g of triethanolamine and 13 g of methanol was added.This mixture was heated for another hour at 60° C. and then cooled toroom temperature to give 332 g of a pale yellow liquid containing 6% Zrand 1.45% B.

Example 3

A 500-ml flask was charged with 100 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate). Agitation was started and 135.3 g oftriethanolamine were added. The resulting mixture was heated to 60° C.and held at this temperature for 2 hours. Then, a mixture of 10.5 g ofwater and 66.3 g of tetra-hydroxyisopropylethylenediamine (QUADROLpolyol) was added. This mixture was held at 60° C. for an additional 2hours. Then, a slurry of 14 g of boric acid in a mixture of 34 g oftriethanolamine and 105.9 g of n-propanol was added. This mixture washeated for another 2 hours at 60° C. and then cooled to room temperatureto give 465 g of a pale yellow liquid containing 4.4% Zr and 0.54% B.

Example 4

A 500-ml flask was charged with 60 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate). Agitation was started and 40.6 g oftriethanolamine were added. The resulting mixture was heated to 60° C.and held at this temperature for 2 hours. Then, a mixture of 6.3 g ofwater and 39.8 g of tetra-hydroxyisopropylethylenediamine (QUADROLpolyol) was added. This mixture was held at 60° C. for an additional 2hours. Then, a slurry of 16.8 g of boric acid in a mixture of 40.8 g oftriethanolamine and 34.1 g of n-propanol was added. This mixture washeated for another 2 hours at 60° C. and then cooled to room temperatureto give 238 g of a pale yellow liquid containing 5.2% Zr and 1.25% B.

Example 5

A 500-ml flask was charged with 60 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate). Agitation was started and 81.2 g oftriethanolamine were added. The mixture was heated to 60° C. and held atthis temperature for 2 hours. Then, a mixture of 6.3 g of water and 19.9g of tetra-hydroxyisopropylethylenediamine (QUADROL polyol) was added.This mixture was held at 60° C. for an additional 2 hours. Then, aslurry of 16.8 g of boric acid in a mixture of 40.8 g of triethanolamineand 34.1 g of n-propanol was added. This mixture was heated for another2 hours at 60° C. and then cooled to room temperature to give 259 g of apale yellow liquid containing 4.8% Zr and 1.15% B.

Example 6

A 500-ml flask was charged with 60 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate). Agitation was started and 81.2 g oftriethanolamine were added. The mixture was heated to 60° C. and held atthis temperature for 2 hours. Then a mixture of 6.3 g of water and 39.9g of tetra-hydroxyisopropylethylenediamine (QUADROL polyol) was added.This mixture was held at 60° C. for an additional 2 hours. Then, aslurry of 16.8 g of boric acid in a mixture of 40.8 g of triethanolamineand 34.1 g of n-propanol was added. This mixture was heated for another2 hours at 60° C. and then cooled to room temperature to give 279 g of apale yellow liquid containing 4.45% Zr and 1.07% B.

Example 7

A 500-ml flask was charged with 50 g of tetra-n-propylzirconate (TYZORNPZ organic zirconate). Agitation was started and 145.5 g oftriethanolamine were added. The resulting mixture was heated to 60° C.and held at this temperature for 2 hours. Then, a mixture of 6.3 g ofwater and 19.9 g tetra-hydroxyisopropylethylenediamine (QUADROL polyol)was added. This mixture was held at 60° C. for an additional 2 hours.Then, a slurry of 14 g of boric acid in a mixture of 17.6 g water, 34 gof triethanolamine and 10.9 g of n-propanol was added. This mixture washeated for another 2 hours at 60° C. and then cooled to room temperatureto give 259 g of a pale yellow liquid containing 3.8% Zr and 0.9% B.

TABLE 2 Compositions of Examples NPZ, g TEAZ, g First TEA, g Polyol, gWater, g B.A., g Alcohol, g Second TEA, g Example % Zr Zr, ml (moleratio) (mole ratio) (mole ratio) (mole ratio) (mole ratio) (mole ratio)(Alcohol) (mole ratio) 1 6.00 0.49 167 (1)  0 33 (7.6)  30 (2)  65.4(MA)   72 (2) 2 6.00 0.49 167 (1)  0 33 (7.6)  45 (3)  14.4 (MA)  108(3) 3 4.40 0.67 100 (1)  135.2 (4)  66.3 (1) 10.5 (2.57)  14 (1) 105.9(nPA)   34 (1) 4 5.20 0.57 60 (1) 40.6 (2) 39.8 (1) 6.3 (2.57) 16.8(2)    34.1 (nPA) 40.8 (2) 5 4.80 0.61 60 (1) 81.2 (4) 19.9 (0.5) 6.3(2.57) 16.8 (2)    34.1 (nPA) 40.8 (2) 6 4.50 0.66 60 (1) 81.2 (4) 39.8(1) 6.3 (2.57) 16.8 (2)    34.1 (nPA) 40.8 (2) 7 3.80 0.78 50 (1)  145.5(8.6) 17.6 (8.6)  14 (2)  10.9 (nPA)   34 (2)

TABLE 3 Performance of Examples Fann Time Example Max, min. Cp @ Max. Cp@ 90 min. 1 6.5 585 382 2 7.5 472 370 3 10 500 326 4 9 495 320 5 8.5 672445 6 9.5 420 302 7 7.5 585 418

The cross-linkers produced according to the process of this invention,as prepared in Examples 1-7 are described in Table 2. Thesecross-linkers were tested under identical conditions to those of theComparative Examples. The results are provided in Table 3. Headings andabbreviations used in Tables 2 and 3 are described above.

Table 3 shows that the cross-linking compositions of this inventioncross-link in the desirable 3-10 minute range for use in wells having a250-400° F. (121-204° C.) temperature range and maintain significantlyhigher viscosities (302-445 Cp) than the Comparative Examples (150-228Cp) long enough (at least 90 minutes) to allow successful completion ofthe fracturing operation.

By varying the ratio of components, such as triethanolamine andtetra-hydroxyisopropylethylenediamine, the rate of cross-linking can bevaried to give a faster or slower rate of cross-linking, withoutdramatically decreasing viscosity development or retention. Based onthese observations, the cross-linker solutions prepared according to theprocess of this invention and the cross-linking compositions of thisinvention cross-link in the desired 3-10 minute range and retainsufficient viscosity under high temperature conditions temperature testconditions desired by the oil field service companies.

In addition, each of the Examples of the invention resulted in asolution that was stable for at least 6 months.

1. A process for preparing a solution of a borozirconate complexsuitable for use in a cross-linking composition which comprises: (a)contacting a zirconium complex with a first alkanolamine at a ratio of 2to 10 moles of the first alkanolamine per mole of zirconium in analcohol to form a first mixture; (b) contacting the first mixture withwater at a ratio of about 2 to 10 moles of water per mole of zirconiumand with 0 to 1 moles of a hydroxyalkylene diamine per mole of zirconiumto form a second mixture; (c) contacting the second mixture with asolution of a boron compound and a second alkanolamine in an alcohol, ata ratio of 1 to 4 moles of boron per mole of zirconium and 1 to 4 molesof the second alkanolamine per mole of boron and at a temperature of 25°C. to 90° C. for a period of time sufficient to stabilize the resultingborozirconate solution.
 2. The process of claim 1 wherein 0.1 to 1 moleof a hydroxyalkylene diamine is added in step (b) per mole of zirconiumto form a second mixture.
 3. The process of claim 2 wherein 0.5 to 1mole of a hydroxyalkylene diamine is added in step (b) per mole ofzirconium to form a second mixture.
 4. The process of claim 2 whereinthe hydroxyalkylene diamine isN,N,N′,N′-tetrakis-(2-hydroxyisopropyl)ethylene diamine.
 5. The processof claim 2 wherein the zirconate complex is a tetraalkyl zirconateselected from the group consisting of tetra-isopropyl zirconate,tetra-n-propyl zirconate, and tetra-n-butyl zirconate.
 6. The process ofclaim 5 wherein the first alkanolamine is selected from the groupconsisting of triethanolamine, tri-n-propanolamine,tri-isopropanolamine, diisopropanolamine, and mixtures of two or morethereof.
 7. The process of claim 5 wherein the second alkanolamine isselected from the group consisting of triethanolamine,tri-n-propanolamine, tri-isopropanolamine, diisopropanolamine, andmixtures of two or more thereof.
 8. The process of claim 5 wherein thefirst alkanolamine and the second alkanolamine are each triethanolamine.9. The process of claim 5 wherein the boron compound is selected fromthe group consisting of boric acid, alkali metal borates, alkaline earthmetal borates, and polymeric borate compounds.
 10. The process of claim8 wherein the boron compound is boric acid.
 11. A cross-linkingcomposition which comprises an aqueous liquid; a pH buffer; across-linkable organic polymer; and a borozirconate solution prepared bya process which comprises: (a) contacting a zirconium complex with afirst alkanolamine at a ratio of 2 to 10 moles of the first alkanolamineper mole of zirconium in an alcohol to form a first mixture; (b)contacting the first mixture with water at a ratio of about 2 to 10moles of water per mole of zirconium and with 0 to 1 moles of ahydroxyalkylene diamine per mole of zirconium to form a second mixture;(c) contacting the second mixture with a solution of a boron compoundand a second alkanolamine in an alcohol, at a ratio of 1 to 4 moles ofboron per mole of zirconium and 1 to 4 moles of the second alkanolamineper mole of boron and at a temperature of 25° C. to 90° C. for a periodof time sufficient to stabilize the resulting borozirconate solution.12. The composition of claim 11 wherein 0.1 to 1 mole of ahydroxyalkylene diamine is added in step (b) per mole of zirconium toform a second mixture.
 13. The composition of claim 12 wherein theaqueous liquid is selected from the group consisting of water, aqueousalcohol, and aqueous solution of a clay stabilizer.
 13. The compositionof claim 12 wherein the aqueous liquid is water, aqueous methanol,aqueous ethanol, an aqueous solution of potassium chloride, an aqueoussolution of tetramethylammonium chloride, or a combination of two ormore thereof.
 14. The composition of claim 13 wherein the organicpolymer is a solvatable polysaccharide and is selected from the groupconsisting of gums, gum derivatives and cellulose derivatives.
 15. Thecomposition of claim 14 wherein the organic polymer is hydroxyethylguar,hydroxypropylguar, carboxyethylhydroxyethylguar,carboxymethylhydroxypropylguar, or carboxymethylguar.
 16. A method forhydraulically fracturing a subterranean formation, which comprisesintroducing into the formation at a flow rate and pressure sufficient tocreate, reopen, and/or extend one or more fractures in the formation, across-linking composition comprising an aqueous liquid; a pH buffer; across-linkable organic polymer, and a solution of a borozirconatecomplex wherein the solution is prepared by a process comprising: (a)contacting a zirconium complex with a first alkanolamine at a ratio of 2to 10 moles of the first alkanolamine per mole of zirconium in analcohol to form a first mixture; (b) contacting the first mixture withwater at a ratio of about 2 to 10 moles of water per mole of zirconiumand with 0 to 1 moles of a hydroxyalkylene diamine per mole of zirconiumto form a second mixture; (c) contacting the second mixture with asolution of a boron compound and a second alkanolamine in an alcohol, ata ratio of 1 to 4 moles of boron per mole of zirconium and 1 to 4 molesof the second alkanolamine per mole of boron and at a temperature of 25°C. to 90° C. for a period of time sufficient to stabilize the resultingborozirconate solution.
 17. The method of claim 16 wherein thetemperature in the formation is 275-325° F. (135-163° C.) and wherein,in the process to prepare the solution of borozirconate complex, 0.1 to1 mole of a hydroxyalkylene diamine per mole of zirconium is added instep (b) to form a second mixture.
 18. The method of claim 17 whereinthe solution of borozirconate complex and the cross-linkable polymer arecontacted prior to their introduction into the formation.
 19. The methodof claim 17 wherein the subterranean formation is penetrated by awellbore; a base gel is prepared by mixing the cross-linkable organicpolymer with the aqueous liquid; the solution of borozirconate complex,the base gel, or both further comprise a pH buffer; the solution ofborozirconate complex is contacted with the base gel in the wellbore toproduce a cross-linked gel, and the cross-linked gel is introduced intothe formation from the wellbore.
 20. The method of claim 17 furthercomprising introducing a cross-linking composition comprising thesolution of borozirconate complex, a cross-linkable organic polymer andproppant into the fracture or fractures.
 21. A method for selectivelyplugging permeable zones and leaks in subterranean formations whichcomprises introducing into the permeable zone or the site of thesubterranean leak, a cross-linking composition comprising an aqueousliquid; a pH buffer, a cross-linkable organic polymer; and an aqueoussolution of the borozirconate complex comprising an aqueous liquid; a pHbuffer; a cross-linkable organic polymer, and a solution of aborozirconate complex wherein the solution is prepared by a processcomprising: (a) contacting a zirconium complex with a first alkanolamineat a ratio of 2 to 10 moles of the first alkanolamine per mole ofzirconium in an alcohol to form a first mixture; (b) contacting thefirst mixture with water at a ratio of about 2 to 10 moles of water permole of zirconium and with 0.1 to 1 moles of a hydroxyalkylene diamineper mole of zirconium to form a second mixture; (c) contacting thesecond mixture with a solution of a boron compound and a secondalkanolamine in an alcohol, at a ratio of 1 to 4 moles of boron per moleof zirconium and 1 to 4 moles of the second alkanolamine per mole ofboron and at a temperature of 25° C. to 90° C. for a period of timesufficient to stabilize the resulting borozirconate solution.
 22. Themethod of claim 21 wherein the temperature in the formation is 275-325°F. (135-163° C.) and wherein, in the process to prepare the solution ofborozirconate complex, 0.1 to 1 mole of a hydroxyalkylene diamine permole of zirconium is added in step (b) to form a second mixture.